Tech Section: Notes on Model 116
There is an undocumented circuit change in later versions of Model 116 receivers. The change is in the second detector circuit of the receiver, and also includes the volume control circuit. These changes were not documented in Philco's "Changes In Models" reports issued at the time.
A schematic of the Model 116PX radio-phonograph was recently discovered that shows the changed circuitry. This complete schematic of the 116PX has been posted here.
Let us examine the changes in detail. As shown in the partial schematic at left, earlier production of Model 116 used a simple second detector circuit, with a Type 37 connected as a diode by using the grid as a diode plate and the plate connected to the cathode.
The 37 tube performs the dual function of detecting the audio signal and acting as the AVC diode. The AVC voltage, which appears as a negative voltage across resistor (70), is fed through a 2 megohm resistor (45) to the control grids of the set's RF and 1st IF tubes.
The volume control circuit is a bit unusual. The audio signal is controlled by the wiper arm of potentiometer (68) and sent to the grid of the 1st audio tube, a Type 77, through resistor (67), 500K, and capacitor (65), 0.05 uF. Normally, the audio signal would be coupled from the wiper arm of the potentiometer (volume control) through a coupling capacitor - no resistor - to the grid of the audio amplifier tube.
It should also be mentioned that the suppressor grids of the RF and 1st IF tubes are both grounded in the early version.
Now let us take a look at the later version's circuitry, as shown at right.
Simple things first: In the later version, the audio is coupled from the volume control to the grid of the 1st audio tube in a conventional manner, through a 0.01 uF capacitor (65).
Notice the 2nd detector tube. There is a 110 pF capacitor connected between the plate and grid of this tube, not present in the early version. In this manner, the Type 37 is used as a double diode with plate and grid both used as diode plates.
The grid-cathode combination of the Type 37 detects the audio signal, while the plate-cathode combination acts as a separate AVC diode.
AVC voltage runs from the plate of the Type 37 to where it splits at resistors (45) and (81A), both 1 megohm. As shown above, resistor (45) carries the AVC line to the control grids of the RF and 1st IF tubes, as in the early version.
Now another major difference in the later version: Resistor (81A) carries an AVC signal to the suppressor grids of the RF and 1st IF tubes. The suppressor grids of these two tubes are grounded in the early version.
Final notes: The 116PX schematic, dated November 1, 1935, shows a 2000 ohm resistor (80) connected in parallel with the set's shadowmeter (78), which is also shown in the early version's schematic. Philco issued a production change on November 1, 1935, in which resistor (80) was removed from the circuit. Therefore, later versions with the different 2nd detector/volume control circuit should not have a resistor in parallel with the shadowmeter.
Hopefully this article, along with the 116PX schematic, will be of assistance to you when you need to service a later version of Model 116.